The major purpose of this research is to clarify conflicting findings regarding early predictors of risk for language impairment using a sample of sufficient size to identify subgroups of late talkers that are large enough to make meaningful comparisons. Additional aims include pushing back the age at which early identification of risk for continued delay can be made and providing longitudinal comparative descriptions of the development of language comprehension, language production, and gesture production in late and normally developing children in order to establish a clearer picture of the different paths taken to normal and abnormal language abilities. Both longitudinal and cross sectional studies using the MacArthur Communicative Development Inventories (CDIs, Fenson et al., 1993), and behavioral measures of language, gesture, and memory are proposed to examine these applied issues and theoretical issues related to linguistic modularity. One thousand (1000) children will be sampled using the CDIs at 10, 13, and 16 months of age. Late talkers will be identified at 16 months of age: Late Producers (LP) will be in the lowest tenth percent for vocabulary production but normal in comprehension; I-ate Comprehenders (LC) will be in the lowest tenth percent for vocabulary comprehension and production. A total of 250 toddlers including 50 LP, 50 LC, 50 children matched to the LPs for vocabulary production (PM), 50 children matched to the LC for vocabulary comprehension (CM), and 50 age-matched controls (AM) will continue to fill out the CDI at 20, 24, 28, and 36 months of age. The additional 750 children will receive the CDI again at 28 months of age and a second LP group, one in which comprehension is not specified, will be identified. These data will be used for a separate test of predictors of risk. Twenty (20) children from the 50 in each group (defined above) will be brought into the laboratory for behavioral testing at 16, 20, 28, and 36 months of age. The proposed experiments are designed to identify early predictors of risk for language delay, describe trajectories of development and stability of delay over time for delayed and normal children, explore relationships between language and nonlinguistic cognition (specifically gesture production and aspects of memory), and to validate the experimental classification of the subjects using standardized tests at appropriate ages. By validating CDI and experimental behavioral results against standardized tests and providing rapid feedback to the participating pediatricians regarding their patients' development, this project will also take a major step in raising pediatricians' awareness of early communicative disorders and providing them with a means to make appropriate early referrals.